Vacuum Flask Thermos Bottle

Vacuum flasks, two steel walls with nothing in between

A vacuum flask thermos bottle is a container built as two stainless steel skins with the air pumped out of the gap between them. That empty gap is what does the work, and it is the reason a flask holding half a litre feels barely warm on the outside while the drink inside stays hot. The range covers sports designs with a narrow drinking spout, wide-mouth versions for food, and larger bottles for a full day out.

How does a vacuum flask keep a drink hot?

Heat leaves a liquid in three ways, and the construction blocks two of them. Conduction and convection both need matter to travel through, so removing the air between the walls removes the path. The inner wall is usually mirrored to reflect radiated heat back into the drink, and the remaining leak is the neck and the lid, where the two walls have to join. That is why the lid matters as much as the body, and why a half full flask cools faster than a full one.

Which size suits which day?

Half a litre is one generous mug or two small ones, fits a rucksack side pocket and is the size that gets carried without thinking about it. A litre serves two people at a viewpoint or one person across a full day, and it is heavy enough to notice on a long walk. Beyond that it stops being personal kit and becomes something shared from a car boot. Cold drinks follow the same logic, which is why the same construction turns up on an insulated flask carried in summer.

What should you check on the lid and the body?

• Lid type: a screw stopper with a cup on top pours neatly, while a push-button spout drinks one handed but has more parts to seize.
• Mouth width: a wide opening takes soup, ice cubes and a bottle brush; a narrow one pours without spilling.
• Seal material: removable silicone gaskets can be taken out, cleaned and replaced, which is where flasks usually start to smell.
• Base finish: a rubberised or recessed base stops the flask sliding off a folding table and dents less on rock.

Can the same flask handle coffee, soup and cold drinks?

Physically yes, but flavour travels. Coffee oils and soup fat cling to the seal and to the threads rather than to the steel, and they turn up in the next drink days later. Anyone alternating between the two is better off keeping a flask for savoury food alone, or stripping the lid and washing the gasket separately. For plain water on the move, a lighter single-wall bottle from the hiking water bottles range weighs less for the same volume. A bottle with filter water answers a different problem again, which is refilling from a stream.

How do you clean and store one?

Wash by hand with a long brush and leave the lid disassembled to dry, since a stopper screwed onto a damp flask is what breeds the stale smell people blame on the steel. Bicarbonate of soda and warm water lift tea staining from the inner wall without scouring it. Store it open, upside down, with the lid alongside rather than screwed on.